The focus of the experimentation is on an elucidation of the developmental lineage of osteogenic cells, in particular, on osteoblasts. We have generated a detailed cellular and molecular model of long bone formation in the embryonic chick limb to assist in focusing our experimentation. Our working hypothesis is that there is a lineage of osteogenic cells which is continuous and progressive; osteoblasts exhibit different phenotypic properties as a function of their place along this lineage and the site in which they are involved in bone formation. As experimentation to test this working hypothesis, we propose to isolate specific monoclonal antibody probes to osteoblasts and bone matrix. In addition, a detailed light and electron microscopic analysis including immunohistochemistry is proposed as well as a detailed study of the proteoglycans synthesized by osteogenic cells along their lineage pathway. The basis for comparative study of osteoblasts resides in the fact that osteoblasts at different anatomical locations and at different developmental times seem to be involved in bone formation with different functional characteristics. Thus, a comparison of osteoblasts from first tibial bone (periosteal bone), from endochondral bone at the growth plate, and from intramembranous forming bone of calvaria will provide insight with regard to phenotypic variations of osteoblasts and lineage characteristics of these cells. Our working hypothesis is testable because osteoblasts can be isolated from various sources and studied in cell culture as well as in vivo. To determine whether these osteoblasts are developmental variants and non-identical requires the availability of cell-specific probes, morphological and histochemical information, and detailed biochemistry of extracellular matrix materials such as bone-specific proteoglycans. All of these avenues of experimentation are proposed here and form the basis for our continuing exploration into the development and maturation of bone.